Packaging materials used for packaging food products or pharmaceutical products require properties (gas barrier properties) to protect the product inside from moisture, oxygen and other gases that degrade the product in order to prevent the product from being degraded or decaying and retain the functions and nature of the product.
Accordingly, conventional packaging materials have been provided with a gas barrier layer made of a material having gas barrier properties. The gas barrier layer has been formed on a substrate such as a film or paper by sputtering, vapor deposition, wet coating or printing. Further, the gas barrier layer has been formed of a metal foil or a metal deposition film made of a metal such as aluminum, or a resin film made of a material such as polyvinyl alcohol, ethylene-vinyl alcohol copolymer or polyvinylidene chloride (e.g., see PTLs 1 to 5).
Although metal foils and metal deposition films have good gas barrier properties, they have various problems, for example, that recognition of products is not possible due to the opaqueness of the metal foil or metal deposition film, cracks occur with only several percent of elongation due to the low elasticity, thus leading to lowering of the gas barrier properties, and the package has to be treated as an incombustible material when discarded after use.
Further, although gas barrier layers made of a polyvinylidene chloride resin film have good gas barrier properties without humidity dependence, they contain chlorine and may become a source of generating harmful substances such as dioxins during disposal treatment. As a consequence, gas barrier layers made of a resin film are unpopular for use as packaging materials.
On the other hand, gas barrier layers made of a resin film of non-chlorine polyvinyl alcohol or ethylene-vinyl alcohol copolymer have high gas barrier properties in a low-humidity atmosphere. However, since they are humidity-dependent, they have disadvantages that the gas barrier properties significantly decrease with increase in humidity.
Other types of gas barrier resin films are inferior in gas barrier properties to polyvinylidene chloride resin films or polyvinyl alcohol resin films that are placed in a low-humidity atmosphere.
In order to improve gas barrier properties of these resin films, there has been proposed a resin film that is made of a composite of polyvinyl alcohol or ethylene-vinyl alcohol copolymer and an inorganic layered mineral. In such a resin film, the inorganic layered mineral should be orderly distributed and arranged in the film so as to improve gas barrier properties. However, the regular distribution and arrangement of the inorganic layered mineral will lead to lowering in cohesiveness of the resin film or adhesiveness to the substrate. Consequently, it is quite difficult to balance high gas barrier properties with sufficient laminate strength as a packaging material.
In view of the above, a gas barrier film has been proposed in which an aqueous polyurethane resin having water solubility or water dispersibility is added to a composite made of polyvinyl alcohol or ethylene-vinyl alcohol copolymer and a layered compound so as to improve adhesiveness to the substrate (e.g., see PTL 6).
This gas barrier film has good gas barrier properties in a high-humidity atmosphere and good adhesiveness to the substrate. However, due to the inferiority in cohesive strength, the gas barrier film exhibits poor laminate strength when laminated to another film for use as a packing material.